Color responses of novel receptors for AcO- and a test paper for AcO- in pure aqueous solution

Article abstract of DOI:10.1016/j.tetlet.2007.10.034

An efficient AcO^- sensor L which contains 1,10-phenanthroline-based and nitrophenylhydrazine-based groups, and its water-soluble Ru(II) complex were synthesized, characterized and studied in this Letter. Via UV-vis experiments and ¹H NMR titration in DMSO solution, it was found that there were potential hydrogen bonds between the N{double bond, long}CH and AcO^- after the deprotanation of two -NH during the reaction of L or its Ru complex with anions. Furthermore, an easy-to-prepare test paper was developed to detect AcO^- at 10 mg/L in pure aqueous solution.

Full text of DOI:10.1016/j.tetlet.2007.10.034

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Tetrahedron Letters 48 (2007) 8615–8618
Color responses of novel receptors for AcOꢀ and a test
paper for AcOꢀ in pure aqueous solution
Xudong Yu,a Hai Lin,b Zunsheng Caia and Huakuan Lina,*
aDepartment of Chemistry, Nankai University, Tianjin 300071, PR China
bKey Laboratory of Functional Polymer Materials of Ministry of Education, Nankai University, Tianjin 300071, China
Received 13 June 2007; revised 2 October 2007; accepted 6 October 2007
Available online 10 October 2007
Abstract—An efficient AcOꢀ sensor L which contains 1,10-phenanthroline-based and nitrophenylhydrazine-based groups, and its
1
water-soluble Ru(II) complex were synthesized, characterized and studied in this Letter. Via UV–vis experiments and H NMR
titration in DMSO solution, it was found that there were potential hydrogen bonds between the N@CH and AcOꢀ after the deprot-
anation of two –NH during the reaction of L or its Ru complex with anions. Furthermore, an easy-to-prepare test paper was devel-
oped to detect AcOꢀ at 10 mg/L in pure aqueous solution.
ꢀ 2007 Elsevier Ltd. All rights reserved.
The design and synthesis of efficient sensors that can rec-
ognize and sense anions selectively through the means of
electrochemical and optical responses is currently an
increasing field in supramolecular chemistry due to their
important roles played in biological, industrial, and
environmental process.1 It is well known that amides,2
(thio)ureas,3 ammonium,4 imidazole,5 and imidazolium6
are particularly effective in the reaction with anions
through the formation of hydrogen bonds between the
active N–H group and the anions, and there are many
receptors of them with molecular clefts or cavities,
which can discern anions with different geometries. Fur-
thermore, with the strong inducement of the anions,
proton transfer often takes place during the host–anion
is introduced for visual anion detection, moreover, the
introduction of Ru containing group not only strength-
ens the affinity of the host with anions, but also makes it
N
N
N
N
HN
H
H
NH
NO2
NO2
L
reactions.7 Compared with the Fꢀ and H2PO sensors,
ꢀ
4
AcOꢀ sensors are limited for most of the AcOꢀ recep-
tors usually have a stronger affinity for Fꢀ. Here, we
HN
NO2
report an AcOꢀ sensor L which contains 1,10-phenan-
+2
N
throline-based
and
p-nitrophenylhydrazone-based
2þ
C
groups, and its cis-RuðbipyÞ2
complex which have
N
high selectivity for AcOꢀ. Moreover, based on the high
selectivity and water-solubility of this Ru(II) complex,
we prepare an AcOꢀ test paper for practical application.
H
N
N
Ru
N
N
H
C
N
This novel hydrozone L is synthesized by the 4,7-dicarb-
oxaldehyde-1,10-phenanthroline and p-nitrophenylhydr-
azine. As a good colorific group, p-nitrophenylhydrazone
N
N
H
NO2
*
Corresponding author. Tel.: +86 022 23502624; fax: +86 022
23502458; e-mail: hklin@nankai.edu.cn
cis-Ru(bipy)22+ complex
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.10.034

8616
X. Yu et al. / Tetrahedron Letters 48 (2007) 8615–8618
water-soluble and able to detect the anions in pure
1
water. By the UV–vis experiments and the H NMR
titrations, it is observed that L and its Ru(II) complex
have high selectivity for AcOꢀ with dramatic color
changes: L has the color change from faint yellow to
green, and the Ru complex has the color change from
purple to green with the addition of AcOꢀ.
2þ
Figure 4. Color change of cis-RuðbipyÞ2
(4 · 10ꢀ5 M) with AcOꢀ (4 · 10ꢀ5 M).
complex mixing
From an overall consideration of solubility and polarity,
the UV–vis experiments of the hosts with anions were
performed in DMSO solution at 298.2 0.1 K. As
shown in Figure 1 of L with different anions as TBA
salts, the intensity of the absorption peak at 434 nm
was remarkably decreased with a simultaneous growth
of a new peak at 622 nm as the anion concentration in-
creased, which was accompanied by the color change
from yellow to green (Fig. 3). The spectral changes dur-
ing the AcOꢀ addition was more obvious than that of
Fꢀ and H2PO ꢀ. In addition, from Figure 2, the Ru
4
complex titration with AcOꢀ exhibited a broad strong
absorption peak at 430 nm with a shoulder at about
463 nm, which was assigned to the metal-to-ligand
2þ
charge transfer (MLCT) of RuðbipyÞ2 moiety coupled
with the p-nitrophenylhydrozone centered charge trans-
fer (CT).8 Also, a new peak at 696 nm appeared and in-
creased with the color change from purple to green
(Fig. 4) upon the addition of AcꢀOꢀ. The addition of ba-
sic anions such as Fꢀ or H2PO also triggered the sim-
4
Cl-
I-
1.4
Br-
Host
ilar but more weaker absorption spectral changes for the
cis-RuðbipyÞ2 complex compared to AcOꢀ. However,
2þ
1.2
other anions including Clꢀ, Brꢀ, Iꢀ did not cause notice-
able spectral changes or color changes, even in the pres-
ence of a large excess of them. The job-plot experiments
revealed that the L and its Ru(II) complex both reacted
with the AcOꢀ to form 1:1 complexes (via a total con-
centration of 4.0 · 10ꢀ4 M). By nonlinear least-squares
fitting9 at kmax = 696 nm, the association constants Ka
of L, Ru(II)complex with AcOꢀ are 3.91 · 104,
8.96 · 105, respectively, according to the following
equation:
1.0
0.8
0.6
0.4
0.2
0.0
AcO-
F-
-
H2PO4
300 400 500 600 700 800
Wavelength
Figure 1. UV–vis titration of ligand L in DMSO (4 · 10ꢀ5 M) solution
upon the addition of different anions (1 · 10ꢀ4 M).
X ¼ X0 þ ðXlim ꢀ X0ÞfcH þ cG þ 1=Ka
1=2
ꢀ ½ðcH þ cG þ 1=KaÞ2 ꢀ 4cHcGꢁ g=2cH
[AcO-]/[Ru complex]
7.5
2.0
To further investigate the anion–sensor reaction mecha-
5
1
1.8
nism, H NMR titration was carried out. From Figure
3
2
1.5
1
0.9
0.75
0.6
0.5
0.4
0.3
0.2
0
5, before the 2 equiv of AcOꢀ was added, the peak of
H1 (–NH) was broad and appeared downfield, while
other Hs did not show any obvious changes
(a ! b ! c). Therefore, it showed that there was a
hydrogen bond between –NH and AcOꢀ. After two or
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
H2
300 400 500 600
Wavelength
700 800
e
d
Figure 2. UV–vis titration of Ru complex in DMSO (4 · 10ꢀ5
)
solution upon the addition of AcOꢀ as TBA salts.
c
H1
b
H2
H1
a
13
12
11
10
9
8
7
ppm
Figure 3. Color changes observed for
L
in DMSO solution
Figure 5. Partial 1H NMR (400 MHZ) spectra of host L (2 · 10ꢀ5 M)
in DMSO-d6 upon the addition of ACOꢀ. (a) Free, (b) 1 equiv of
ACOꢀ, (c) 2 equiv of ACOꢀ, (d) 3 equiv of ACOꢀ, (e) 4 equiv of
ACOꢀ.
(1 · 10ꢀ5 M) upon the addition of 10 mol equiv of anions as TBA
salts. From left to right: host, host +ꢀAcOꢀ, host + Fꢀ,
Host þ H2PO4ꢀ, Host + Clꢀ, host + Br, host + I .

X. Yu et al. / Tetrahedron Letters 48 (2007) 8615–8618
8617
more equiv of AcOꢀ were added, the –NH-acetate reso-
nance disappeared, instead, the peak of H2(–CH) had
now clearly downshifted (from 8.82 to 9.29 to
9.48 ppm, c ! d ! e) revealing at higher concentrations
of acetate, the –NH deprotonated. At the same time, the
–N@CH reacted with acetate to form a strong potential
hydrogen bond (when deprotonation happened, the ace-
tate and the acetic acid coexisted in the solution, surely
the acetate was as a base to bind with the –CH not the
acetic acid). At last, the receptor reacts with the anions
to form a 1:1 supramolecular complex (the job plots are
given in the Supplementary data) in the DMSO solu-
tion. The whole reaction process was shown in Figure
6. This phenomenon may be attributed to its basicity
and geometry configuration to converge the two –NH
groups at a point via –NH-anions hydrogen binding that
required twisting the –CH@N–NHAr groups substan-
tially out of the plane of the phenanthroline ring, this re-
sults in some odd bond angles, such that the extended
conjugation will be diminished, and the receptor cannot
have such remarkably high affinities with only two NH
hydrogen bond donor groups. On the contrary, with
the two N@CH as binding sites, the geometry will be
more suitable for the formation of stable N@CH-anions
hydrogen bonds. Furthermore, it will make the electron
deficient phenanthroline have more delocalization space.
The –CH had a chemical shift at 8.817 ppm which made
it possible to bind with the basic anions. The angle of
two oxygen atoms in triangular acetate is 120ꢁ, which
allowed for a better matching of geometry between the
two N@CH compared to other anions such as Fꢀ and
H2PO4ꢀ. The complexation of Ru in L will only act to
intensify this effect, so the Ru complex had more associ-
ation constants with anions than its ligand L.
Figure 7. The color changes of the test papers for detecting acetate ion
in neutral aqueous solution with different NaAcO concentrations.
binding sites of the receptors. For this reason, it cannot
be an efficient sensor for anions in aqueous solution
though it was water soluble. To avoid the competing sol-
vent effect of the water, we prepared a test paper based
2þ
on the water solubility of cis-RuðbipyÞ2
complex.
(This test paper was made by putting a filter paper into
the acetone solution of the Ru complex (10ꢀ4 M) for
24 h and then drying it in the air). This paper was then
placed into acetate solution in water, dried it in the air to
remove water gradually, which resulted in a color
change in the paper. It can detect the presence of minute
AcOꢀ in pure aqueous solution at 10 mg/L (Fig. 7). To
our knowledge, there are many traditional methods such
as 19F NMR spectroscopy for detecting Fꢀ and other
anion recognitions were taken in a mixed solution of
water and an organic solvent. However, there are few
simple methods or instruments to detect AcOꢀ in pure
aqueous solution, we hope that this interesting and
easy-to-prepare test paper will be extensively applicable
in daily life.
In a word, a new hydrozone L and its water-soluble cis-
2þ
RuðbipyÞ2 complex as the colorimetric anion sensors
It is generally accepted that receptors for anions based
on hydrogen binding interactions cannot be served as
efficient sensors in pure aqueous media for the strong
protic solvent competition. When water was added into
the DMSO solution of RuðbipyÞ22þ complex with AcOꢀ,
the green color faded and it indicated that the protic sol-
vent water would compete with the anionic guest for the
were synthesized, characterized and studied in this Let-
ter, they both have high selectivity for AcOꢀ than other
anions such as H2PO4ꢀ, Fꢀ, Clꢀ, Brꢀ, Iꢀ. Furthermore,
an interesting and easy-to-prepare test paper of cis-
RuðbipyÞ2 complex for the detection of AcOꢀ in pure
2þ
aqueous solution was developed for detecting AcOꢀ in
aqueous solution. This test paper successfully resolves
the competition of water–guest for the binding sites.
We hope that this test paper will be put into use in the
future.
NO2
H1N
N
NO2
AcO-
C
H1
H1
H2
N
N
O
N
N
Acknowledgement
O
instable hydrogen bonding
NO2
H2
C
This project was supported by the National Natural sci-
ence Foundation of China (Nos. 20371028; 20671052).
N
N
NO2
H1
Supplementary data
N-
NO2
N
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2007.10.034.
H2
added AcO-
O
N
depronation
-2H
O
N
H2
N-
N
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NO2
Figure 6. The proposed reaction mechanism of the receptor with
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